A new crystal form of bovine heart ubiquinol:cytochrome c oxidoreductase: determination of space group and unit cell parameters.

Edward A. Berry, Vladimir M. Shulmeister, Li-shar Huang, and Sung-Hou Kim

Structural Biology Division

Lawrence Berkeley Laboratory

Keywords: Cytochrome bc1 complex, E.C. 1.10.2.2, membrane protein, crystallization, space group, orthorhombic, unit cell parameters

Summary- Ubiquinol:cytochrome c oxidoreductase, the middle segment of the mitochondrial respiratory chain, is a multi-subunit transmembrane redox enzyme. The purified protein from beef heart mitochondria has been crystallized by three groups in three different crystal forms, but progress toward a structure has been hampered by the limited order (resolution) of the crystals. We have found that under certain conditions our preparation of the enzyme crystallizes in a new form suitable for x-ray diffraction studies. These crystals belong to the space group C2221, in the orthorhombic system. The cell dimensions are 384, 118, and 177 Å. These new crystals at present diffract to 3.8 Å at best. This is not significantly better than our hexagonal (P61(5)22) crystals, but the new crystals have the advantage of less spot overlap due to face centered packing which results in systematic extinctions. More importantly, the availability of the same enzyme in multiple crystal forms may allow phase refinement and extension by the methods of molecular replacement.

Correction:
When indexed as face centered, the angle beta is indistinguishable from a right angle. This led to assignment of the orthorhombic space group, but this is not reflected in the internal symmetry which shows a single crystallographic 2-fold axis. The correct space group is P2(1) and the unit cell parameters are 118.0 178.0 199.8 90. 106. 90. The indices h, k, l in the p2(1) indexing can be calculated from h', k', l' in C222(1) indexing as h k l = k',l',(h'-k')/2.

Like the chicken orthorhombic crystals, these crystals contain a bc1 dimer in the asymmetric unit.